Various types of write heads for charging the surface of a print cylinder are known, for example write heads employing lasers, light-emitting diodes or heat. Also known are microtunnel (M-tunnel)write heads, where the write head induces a strong electrostatic field within a defined volume and forms a microplasma of positive ions for contacting and charging the recording surface. The resultant charges correspond to a desired image which is to be printed, typically after the charged recording surface attracts ink or toner after passing an inking station.
The M-tunnel process is described, for example, in U.S. Pat. No. 5,325,120. In the M-tunnel write head shown in the '120 patent, a small needle-shaped electrode is placed at the top of each of a plurality of microtunnels. This anode accepts positive voltage pulses which generate positive ions. The positive ions are attracted toward a recording surface by a ring-like gate electrode or cathode placed in the tunnel wall near the bottom of the microtunnel, the gate electrode having the voltage to which the recording surface should be charged. A grounded substrate is provided under the dielectric recording surface to further attract the positive ions. The ions accumulate on the recording surface until they reach the voltage potential of the gate electrode, thereby charging the recording surface and permitting the surface to attract ink at a later point in the print process.
The M-tunnel structure described in the '120 patent however has several disadvantages. First, the distance between the ion-generating needle-shaped anode and the write surface creates an electrical resistance which requires a large potential at the anode for desired charges to accumulate on the write surface. Second, the needle-shaped anode may become very hot when generating the plasma. Third, the pixel "footprint" left on the recording surface is strongly defined by the cross-sectional shape of the microtunnel at its exit near the recording surface.